Nowadays, there are known selection criteria which must be satisfied by a prosthetic heart valve. These criteria have been established by the Food and Drug Administration (FDA) which is the organization governing this field in the United States of America.
There are seven such criteria:
1) The first criterion is defined by the blood compatibility of the materials used. These materials must be compatible with the organism and they must behave inertly with respect to natural coagulation processes, i.e. they must not induce thrombosis phenomena on contact. A local thrombosis can give rise to embolism and can even hinder valve operation. Several materials behave satisfactorily from this point of view: specific metal alloys such as chromium, nickel-tungsten and titanium, and above all pyrolytic carbon, which is the material which is used most frequently nowadays in the valve prostheses that are commercially available
2) The second criterion is defined by the quality of the wake induced in the blood flow by the prosthesis. Zones of turbulence, eddies, and stagnant zones all give rise to the same phenomena of thromboses and embolisms, to such an extent that it has been said that the disturbances generated in the blood flow are more likely to generate thrombosis than the material used. That is why biological valves ("bioprotheses") which reproduce the form of the natural flow have a great advantage over mechanical prostheses. Such valves do not require anti-coagulant medication to be taken. These two thrombosis-generating factors (Material and Wake) are still imperfectly mastered on mechanical valve prosthesis, which means that patients must be put on anti-coagulants for life, thereby constituting a major drawback compared with bioprotheses.
Unforunately, that does not mean that biological valves constitute an ideal solution, since clinical experience shows that, unlike mechanical valves, their lifetime is too short. Because of their progressive deterioration, the surgeon often has to replace them at the cost of further major surgery. It has not been possible, in the past, to reproduce the flow characteristics of a natural valve when using a mechanical prosthesis. That is the object of the present invention.
3) The third criterion for mechanical valves is durability: in accelerated fatigue testing these valves must demonstrate that they have a lifetime equivalent to 560 million cycles, i.e. the equivalent of about 15 years under physiological working conditions (at a mean frequency of 70 heart beats per minute). This mechanical lifetime is related to the geometrical design of the valve, to the guide and abutment means adopted for the, or each, valve member or "flap" in the valve, and to the mechanical characteristics of the material used.
4) The fourth criterion is directly related to the second (Wake) and concerns the head loss which a mechanical prosthesis gives rise to on systolic ejection or on diastolic filling of the ventricle. A portion of such head loss is inevitable since it is inherent to the reduction in the effective orifice area (EOA), with the reduction in effective orifice area being related to the reduction in diameter caused firstly by the textile ring which is essential for surgical installation of the valve, and secondly by the thickness of the metal ring which supports the flap(s) and the hinge means enabling the flaps to move between an open position and a closed position. A second portion of the head loss is due to the geometrical disposition of the flap(s) and is mainly due to flap orientation relative to the flow of blood.
With a small anatomic orifice, in particular in a child, this factor is most important. Pressure gradients of more than 40 mm of Hg between the left ventricle and the aorta are thus commonly observed with 19 mm and 21 mm sizes, and this makes additional work for the heart muscle. This pressure gradient may reach higher values when the pumping rate of the heart increases. The surgeon is therefore happier using mechanical valves which, for given size, cause less stenosis.
5) The fifth criterion used concerns the closure time of the valve members when the ventricle contracts or relaxes. It is related to the geometrical definition of the valve and of its flap(s), and it determines the regurgitation fraction which at present, in the best of cases, is at least 5% of the volume of blood set into motion during each cycle. When a patient has two valves on the same ventricle, leakage is at least about 10%, and this puts non-negligible stress on the heart muscle.
6) The sixth criterion is static leakage, i.e. the mechanical sealing of the prosthetic valve when closed. For the same reasons, this criterion also has an influence on the hydraulic efficiency of the valve.
7) The seventh criterion may be considered as being the sum of the other six criteria: it concerns proper operation of the valve in man, as observed over a period of several years with the minimum possible number of thrombo-embolic incidents. A few mechanical valves give quite acceptable results. However, both moribity and mortality remain significant since the number of embolic incidents lies around 2% per annum and per patient, and mortality in many cases is of cardiac origin, and thus directly or indirectly linked to the valve prosthesis.
On this topic, it should be observed that these pathological phenomena are more common with patients having such a valve in the mitral position since the flow of blood through the valve in this configuration takes place at low pressure and for a longer period of time, thereby being more subject to thrombosis in the auricles, particularly since the prosthesis causes more stenosis, more regurgitation, and more turbulence, and is less well sealed.
In the mitral configuration, neither mechanical nor biological valve prostheses give full satisfaction. That is why it is important to improve existing products.
It may be recalled that the first prosthetic valves proposed in the early days of heart surgery, at the beginning of the 60s, were valves that attempted to reproduce the shape of natural heart valves. Because of their complexity and because of the technological possibilities available at that time, these prostheses were not reliable and they had to be abandoned. It was revolutionary when Star and Edwards proposed solving this problem by going right away from the natural model when they invented the valve constituted by a ball retained in a rigid cage. The function of the valve was thus reproduced reliably, albeit crudely. There are some surgeons who are still using this type of valve.
Manifestly the presence of a ball in the flow of blood gives rise to large head loss and to considerable disturbance in the flow. Observations made on man have confirmed this drawback. Later, Bjork proposed replacing the ball by a circular disk pivoting on a hinge device. Head loss was thus reduced. However, for mechanical reasons, it is not possible for the pivoting disk to tilt fully through 90.degree.. The open angle is about 60.degree. to 75.degree.. There is thus a small orifice and a large orifice. It is easy to imagine that such an asymmetrical device has undesirable effects on the wake: there is a large zone of turbulence downstream from the valve over the small orifice, and this zone is responsible for thrombo-embolic phenomena, and, sometimes, for valve malfunction. These anomalies are clearly revealed nowadays by modern flow display techniques (Doppler-anemometry).
From the end of the 70s, a third generation of valve has provided a significant improvement on this point by using two half-disks pivoting on micro-hinges close to the diameter, instead of using a single tilting disk. The opening angle of both valve members comes close to perpendicular. The wake generated thereby is more symmetrical. Head losses are substantially reduced. Valves of this type are now the ones most frequently used in surgery on humans.
In summary, a single disk is better than a ball, and two half-disks (a genuinely "butterfly-like" valve) are better than a single disk. However, in valves of this type, the hinge axes of the valve members are centrally located in the zone of high speed flow, and since the opening angle is never 90.degree. (since that would give rise to too high a regurgitation fraction), thrombosis-generating zones of turbulence remain with an imperfect wake.
U.S. Pat. No. 4,276,658 (corresponding to French patent FR-2-407 709) describes a heart valve prosthesis of the type comprising a body provided with a central blood flow passage, means for controlling this flow of blood, which means (flaps) pivot under the action of the blood flow between a closed position and an open position, and hinge means for the flow-controlling means.
The valve described in these patents includes hinge means comprising: cavities formed in the annular body of the valve presenting a bearing surface in the form of a surface of revolution, and flaps provided with projections intended to penetrate into the above-mentioned cavities with their ends sweeping the surfaces of revolution, each end being in contact with one of the bearing surfaces.
More precisely, the cavities are formed in the internal flat faces of two parallel and diametrically opposite lugs, projecting axially upstream from the periphery of the annular body of the prosthetic valve.
Although the valve of the above-mentioned patents is acceptable given the selection criteria mentioned above, it should nevertheless be observed that it does not reach optimum values of resistance to thrombosis because it has zones in which blood is at least partially stagnant, said zones being constituted by the cavities in which the projections of the flaps rotate. In addition, given that these flaps have only one degree of freedom, defined by rotation about an axis passing through two opposite cavities provided in the above-mentioned lugs, it is inevitable that the sweeping of the cavities is prejudicial to red corpuscles which are crushed therein. Further, the dynamic characteristics of the flow of blood through the valve are degraded by the central disposition of the flaps when in the open position, extending through the high-speed zone.
The Applicants have already proposed a prosthetic heart valve having excellent performance, particularly with respect to thrombosis resistance and to hemodynamics (which therefore satisfies the main two selection criteria for a prosthetic valve as underlined above), while also being highly satisfactory from the point of view of the other parameters defining the above-mentioned selection criteria. This prior valve constitutes the subject matter of French patent FR-2 587 614 and the corresponding U.S. patent application Ser. No. 06/910 621. In particular, it can easily be seen that in the valve of these two patents, account is taken of the influence on turbulence of the disposition of the closure flaps when in the open position, and also of the influence on closure time (i.e. how quickly the flaps close), when the direction of blood flow reverses. This valve also takes account of the anatomic configuration of natural heart valves, which are larger in section on the downstream side, corresponding:
in the mitral position, to the inlet into the ventricle; and
in the aortic position, to the three sinuses.
More precisely, this prior valve takes account of the fact that the blood flow deviates (diverges) towards the wall of the valve in forward flow and that it converges towards the center of the valve in reverse flow.
In addition, the disposition of the flaps in the vicinity of the walls of the valve and the fact that the downstream ends of the flaps close towards the center of the valve clearly present several advantages:
the central portion of the valve where the flow is strongest is completely disengaged;
the flaps can be placed in a position where they lie parallel to the valve axis, thereby taking advantage of the convergent deviation of anatomic origin in reverse blood flow, as mentioned above, for initiating the closure movement; and
since the flaps sweep through substantially the same zones when in motion as are swept through by the replaced natural valve, they run no risk of encountering obstacles to operation; in particular, a flap whose trailing edge opens from the periphery towards the center can run the risk of a possible fold in the tissue in the vicinity of the base preventing the flap opening at all, whereas in our prior valve, such a fold can at worse slightly limit full opening.
Naturally, when replacing an aortic valve, it would be preferable to place a single prosthetic valve with three flaps facing the three sinuses of the natural aortic valve replaced in this way. By placing the hinge axes of the flaps as close as possible to the wall and to the bottom edge of the flaps when in the open position, the flaps can be closed rapidly. Further, the valve includes appropriate means for minimizing wear of the means for guiding the flaps in rotation and for retaining them, thereby improving length of life and reliability of a prosthetic heart valve. These means for minimizing wear in the means for guiding the rotation of the flaps and for retaining them are the following:
maximum possible distance between the abutment point which limit flap motion during opening or closing so as to obtain the motion required for limiting the motion using the smallest possible forces;
increasing the areas of the flaps that come into contact with the base of the valve in the extreme positions so as to obtain the required forces using pressures which are as low as possible (this situation also contributes to reducing operating noise); and
eliminating fixed spots of rubbing between the flaps and the base of the prosthetic valve during motion.
It is also easy to see that the structure of this prosthetic valve is such that all of its portions are accessible to the flow of blood during the course of an operating cycle, thereby avoiding the formation of zones of stagnant blood which encourage the formation of clots. To this end, the valve takes special care at the hinges of the flaps, whereas the problem of zones of stagnation is very poorly solved in most currently existing prosthetic valves because of the configurations adapted for their flat hinge means.
Although the valve constituting the subject matter of French patent FR-2 587 614 and U.S. patent application Ser. No. 06/910 621, as outlined above satisfies the above-mentioned selection criteria very well, in addition to the various other dispositions already mentioned, the present invention seeks to provide a prosthetic heart valve which, while satisfying the dispositions of the earlier valve, also includes means for guiding the rotation of the flaps and for retaining them which, while avoiding the formation of zones of stagnation in the blood flow, additionally have a configuration which lends itself to coating the body of the valve (including its means for guiding the flaps in rotation and for retaining them) with pyrolytic carbon, which is a material that is thrombosis resistant and which withstands wear.